@dipsomaniac I had the same issue with sideloaded epub reboots upgrading from 1.9.17 to 2.0.0. To resolve I had to factory reset ( ouch) remove all books from the library ( easy enough if on an sd card) upgrade to the 2.x.x firmware then add the books to have the library build from scratch. Sideloaded libraries apparently have a bit of a wobble upgrading when you move to 2.x.x. I've had no further issues upgrading to later firmware revisions so I'd see it as a one off bit of bullett biting.